The present invention relates to AM stereophonic signal receivers and more particularly to an AM stereophonic signal receiver which is suitable for receiving signals using either an AM-FM stereophonic technique or an AM-PM stereophonic technique.
In an AM stereophonic broadcast system, it is necessary that the carrier wave be amplitude-modulated with the sum (main) signal of the right and left channel signals so as to be compatible for reception by a monophonic signal receiver. The difference (sub) signal between the right and left channel signal is transmitted for stereophonic signal reproduction. Any of an AM/AM technique, an AM/FM technique or an AM/PM technique can be used as the AM stereophonic broadcast technique.
In accordance with the AM/AM technique, transmission is carried out by amplitude modulating with the sub signal a carrier wave which is equal in frequency to and different by 90.degree. in phase from the carrier wave of the main signal. The AM/AM technique is called "an orthogonal modulation technique". In accordance with the AM/FM and AM/PM techniques, a carrier wave is phase angle-modulated (FM or PM) with the sub signal and the modulated carrier wave is amplitude modulated with the main signal prior to transmission.
Especially with the AM/PM and AM/FM techniques, because of the bandwidth limitations of the AM broadcast band, the frequency and phase deviations of the sub signal are limited. This leads to an inherent reduction in the electric field strength of a signal reaching a receiver so that the quality of the reproduced signal at the sub detection output is degraded more than that at the main detection output as a result of which the quality of sound obtained by reproducing the right and left channel signals is highly degraded.
Accordingly, an object of the invention is to provide an AM stereophonic signal receiver in which degradation of the quality of sound due to a decreased electric field strength with the AM/PM and AM/FM techniques is prevented thereby to provide satisfactory reproduction outputs.